Haven’t mustered the energy to tear into it, ruin a super Xmas day but expect to find a piston with a hole melted thru it. Really a bit shocked since seals and gaskets tight and all else in spec. Back soon with pictures so you enduro geniuses can help me with root cause analysis.

Really sucks what happened.

Would be a good idea to do a pressure and vacuum test before tearing down.

It should be ruled out now. Or may never know for sure if it might have been an air leak.

Also curious if you were able to use Yamaha seals or the Buzzard ones?

Well this is a bummer...

I had head off when I bought. Piston and cylinder were in good shape so I set timing with dial gauge and put head back on, moved onto replacing main seals (main bearings nice) rebuilding carb, replacing clutch, all other seals, rebuilt forks, replaced all wheel bearings, seals, cables. Adjusted and bled oil pump, put in gas and rode 300 miles only to have incinerated the piston. I highly recommend the Lucas injector oil - it withstood massive heat.

Tdan I looked back and I got Yamaha main seals from a dude with 7500 100% happy transactions so it will be interesting to see how they held up.

Crank and ron still nice and smooth - I need to split cases since aluminum splatter is needs to be cleaned out and I need to make sure this thing is air tight.

Anyone know a good bike machine shop in OC area? Piston skirt perpendicular to wrist pin measures 3.143” or 79.83mm. Is this std size?

Well, I could be upset but if this is the worst luck I have before I escape 2020 then thank God, right?

MarkT - Yamaha seals from eBay seller

Any advice appreciated

79.83 is 0.17mm under 80mm std which is close to 7 thou clearance, but measured up by the gudgeon probably accounts for it as it tapers up the shirt so should be a lot better nearer the base. Looking at the pics you have the melted piston showing the top ring end at the export port. This can't be happening normally unless the piston is back to front. So i guess that all happened when the piston meted & caught the ring in the port & broke it. Or am a reading the pic totally wrong.
Ok, looking at other pics i see the hole in the piston is very small & considering everything has gone out the exhaust & very little from the actual hole has gone down below, i'd be tempted to turn it up on the handlebars & flush it out upsidedown then new piston & carry on.
You say you set the timing. Being cdi i just wonder how you did that. Without having played with one since i sold mine 30 years ago i'm guessing 2.9 btdc & align marks on pulser & flywheel by moving the backplate not the pulser only. I'm only getting sidetracked on that subject wondering if the flywheel damaged the crank key & moved enough to advance the ignition but keep running. Somewhere you said 300 miles before it incinerated which leads me to think something out of the ordinary has happened.
I see the ring end around at the inlet now in the other pic. I'm woffling--i'll leave it there, Hope ya get it sorted ok.

Sir,

Going with an earlier post about jetting, I'm running a #280 in my RT-3 basically at sea level, and drop to a #270 when we're at Hungry Valley at Cal Dreamin. This with Boysen reeds, head milled .005" and one of Dave Meis' Sleeper Pipes. My bike 'gently' seized up on the stock bore on a long uphill out of Santee (San Diego area), and it then had a #230 main jet. Hope this helps.

Hammer

Well, I wish you had not posted a picture of the two stroke oil that I use. But oh well we all have different luck. It's time for a story. I get a lot of phone calls from people and one of the things that they ask about purchasing a vintage motorcycle they include the words "as a daily rider" normally I'm not sure how to respond because I do not have the old style phone receiver that I could bang on the table to get their attention. For the most part vintage motorcycles are a product of yesterday. Yes, they can be run. They are temperamental. They are expensive. Good grief Charlie Brown I'm pushing my motorcycle again.

a

After you check all things others suggested, check your petcock. The exact thing happened to my SC500, I had a new rebuild, new seals and no air leaks. The problem was a aftermarket petcock too small and not flowing enough fuel at full throttle.

With damage like this, we can be certain that it was not an oil related failure. I run the Walmart TCW-3 in my DT and two other air cooled, oil injected vintage bikes. It's been great. I'm sure the stuff you're using is even better.

I also ride my DT daily. They often take quite a bit of work to get them back to that condition but why not. With the DT360A specifically, this sort of thing can occur without any mechanical problems. They ping hard in stock form, especially at high revs and part throttle. I can only imagine that fuel must have been better back in the early 70's. On today's pump gas they are not okay with cruising at highway speeds. Nothing that can't be fixed though.

When we stop riding them is when they really disappear. Relegated to collections and wrecking yards. They're too much fun to let them go that way.